Out of the Loop (no pun intended)

Hey, its been over 6 years since i studied physics seriously, was letting my mind wonder today when i hit upon an idea unfortunately i cant remember the exact physics involved.

Basically i remember something about being able to create an eletrical current using magnetic fields and ferous metals moving within this field, cant remember the exact requirements or calculations though, could someone please help me?

thanks, and now for my idea (its a gradeous one so feel free to laugh and the impossiblity of it) basically, the magnetic field is earths, the rotation circuit is a massive ring in orbit of the world, thirdly several of the "space elveators" that NASA are on about these days act as power cables to the surface of the planet, result, massive "free" energy source.